A protein antigen--P-antigen--characteristic of Branhamella catarrhalis has been described. Precipitating antibodies against this antigen occur in a majority of healthy human sera; serological evidence for a pathogenic role of B. catarrhalis is accumulating. The occurrence of complement-fixing antibodies has been demonstrated during bronchopulmonary infection with B. catarrhalis and in maxillary sinusitis. Increases in antibody titres assessed by an enzyme-linked immunosorbent assay (ELISA) technique have been reported in children with acute otitis media. An ELISA, using purified P-antigen, for the evaluation of IgG, IgM and IgA antibodies against B. catarrhalis is being developed.